LIDAR – Light Detection and Ranging

Laser altimeters or light detection and ranging (LIDAR)
systems, which are active optical sensors that generate energy using lasers,
are also being developed for extremely high-resolution topographic mapping. The
basis of LIDAR sensing is simple. The time interval between when the laser
pulse is generated and received back at the antenna after reflecting from a
target on the ground is recorded. With precise knowledge of the aircraft
position computed using the Global Positioning System (GPS), topography can
then be inferred very accurately. As an active sensor, LIDAR can be
flown at night, but unlike SAR, it is
unable to penetrate clouds.

The Optech ALTM Instrument
installed in a Cessna 206

The LIDAR system combines a pulsed, solid-state laser,
an inertial motion unit (IMU), and a geodetic GPS receiver in a compact and
modular configuration.The IMU
(accelerometers and gyroscopes) monitors the aircraft attitude, while the GPS
receiver provides aircraft position data.Rotating optics in the instrument's sensor head scans the laser across
the ground, illuminating a swath under the aircraft.The LIDAR instrument can be installed on any plane equipped for
aerial photography.Depending upon
aircraft altitude, aircraft velocity, and instrument pulse rate, the LIDAR
instrument can collect data with an illuminated footprint of approximately 15
cm and a swath width ranging from 300 to 900 meters.